1. Field of the Invention
The present invention relates to a method of producing a block mask to be employed in electron-beam lithography apparatuses of a block lithography type. More particularly, this invention is concerned with a method of producing a block mask to be employed in electron-beam lithography apparatuses according to which openings can be created as per designed values. Specifically, when patterns of openings having different sizes are delineated on a block mask by performing lithography, corresponding opening portions of a resist are processed by performing etching. Herein, the openings have a size as per a designed value.
2. Description of the Related Art
The trend of semiconductor integrated circuits is higher-density integration of components due to advances in microprocessing technology and microprocessing technology requirements are becoming more severe. Above all, photolithography, the lithographic technology adopted by existing step-and-repeat lithography systems with reduction or the like, will reach its limits. Electron-beam lithography is a technology that has the potential of overtaking photolithography.
Electron-beam lithography apparatuses occur as variable rectangular lithography types, a block lithography types, and a multi-beam lithography types. The present invention relates to a block lithography type of electron-beam lithography apparatus. Block lithography is a technique for printing graphics. According to the block lithography, a pattern that is a unit of repeated graphics is delineated on a transparent substrate of a mask, an electron beam is transmitted to the transparent substrate in order to delineate the unit pattern at a time, and the unit pattern is linked to a previous pattern. This sequence is repeated to thus print graphics. A pattern of openings is composed of various openings corresponding to patterns for an IC. The openings include, as illustrated, small openings, large openings, rectangular openings, and openings shaped like a combination of rectangles.
According to a method of producing a block mask of a related art, whichever of large openings or small openings are to be created, first, a silicon substrate is coated with a resist. The silicon substrate is then exposed in order to delineate a pattern of openings, and then developed. The silicon substrate is then etched in order to make the openings. Wet etching or plasma etching is adopted as the etching method. At the etching step, an etching rate varies depending on the size of openings. This poses a problem in that if the silicon substrate were exposed proportionally to the size of openings, it would be impossible to create openings having a desired size. If the conditions for etching are determined so that when the silicon substrate is etched in order to create small openings and large openings all together, the small openings may have a desired size but, in this case, the large openings will have dimensions larger than the desired dimensions. Openings in a block mask are required to be very precise in size. Such an error in dimensions is not permissible.
To avoid this problem, it is conceivable to etch a substrate repeatedly for separately creating small openings and large openings. According to this method, lithography and etching must be repeated a plurality of times in order to produce one block mask. This leads to an increase in cost. Moreover, the time required for producing the block mask gets longer. This poses a problem in that the method cannot flexibly cope with the production of any kind of block mask.
Another conceivable method is to adjust an exposure pattern according to the size of an opening and thus delineate a pattern of openings having a desired size. In this method, the conditions for exposure must be calculated for each shape of openings, and thus varied depending on the shape thereof. This poses a problem in that exposure control becomes very complex.